Offshore drilling normally takes place with either a floating drilling rig, a fixed platform, or a jackup drilling rig. A riser or some other type of conduit will extend from the seafloor to the drilling rig. The riser will have a blowout preventer (BOP) that is able to close around a drill string as well as to sever it. The BOP serves to prevent a dangerous blowout of the well in the event an unexpectedly high pressure earth formation is drilled into and overcomes the hydrostatic pressure of the drilling fluid. The BOP may be located subsea near the seafloor or it may be located above sea level at the drilling rig.
Normally the operator drills the well with a string of drill pipe. Drill pipe comprises thick wall joints of pipe that are secured together to make up a string. The drill pipe is constructed so to allow the operator to frequently unscrew and screw the joints together. When the operator reaches a depth that he wishes to run casing, he pulls out the drill pipe, then runs back into the well with the string of casing. The operator cements the casing in the well. The casing may extend to a subsea wellhead assembly or it may extend to a wellhead assembly above sea level at the rig.
In some geographic areas, difficult zones are encountered while drilling the well. For example, a difficult zone may comprise a low pressure, porous zone located below a much higher pressure earth formation or zone. Normally the operator will have the well loaded with drilling mud that has a weight selected so as to be able to prevent the pressure within the higher pressure earth formation from overcoming the weight of drilling fluid and causing the earth formation to flow into the well. If the weight of drilling mud is too low, a blowout might occur. When drilling from a higher pressure zone into a lower pressure zone, the weight of the drilling mud might be too heavy for the lower pressure zone. If too heavy, drilling fluid will flow into the lower pressure zone, resulting in a loss of expensive drilling fluid. Also, circulation may be lost, preventing the drilling fluid from circulating to and from the drill rig. In addition, if the lower pressure zone is intended to be a production zone, the encroaching drilling fluid could irreparably damage the ability of the production zone to produce hydrocarbon.
Operators overcome these problems through experience in estimating where the difficult zones lie. An operator may choose to stop drilling just above the difficult zone, run a string of casing and cement it in the well. The operator then would be able to utilize lesser weight drilling fluid for drilling through the lower pressure zone.
In another technique that has been proposed but is not in widespread use, the operator would run and install casing just above the difficult zone as in the first method. The operator would then lower a liner string with a drill bit on the lower end into the well. The upper end of the liner string would be secured to a string of drill pipe. The operator rotates the drill pipe and the liner string to drill through the difficult zone. Afterward, the operator cements the liner in place. The liner is made up of the same type of pipe as casing, but it does not extend all the way back to the wellhead. Instead, it will be hung off at the lower end of the previously installed string of casing. The term “casing string” on the other hand normally refers to pipe that is cemented in the well and extends all the way back to the wellhead.
While liner drilling as described is feasible, an operator may prefer to have casing extending all the way back to the wellhead. Casing drilling is a known technique that is principally used on land wells. The operator rotates the casing string with a casing gripper mounted to a top drive at the drill rig. A drill bit assembly, which may be retrievable or not, is located at the lower end of the casing string. While this technique works well on land, there are regulations for offshore drilling that restricts this technique. In some geographic areas, regulations state that the blowout preventer for an offshore drilling rig has to be capable of completely severing any drill string passing through it while drilling is taking place. In an emergency, the operator has to be able to close the upper end of the well at the BOP, even if that includes severing the drill string in the well. BOPs used offshore are capable of severing conventional drill pipe. However, BOPs used on offshore rigs are typically not capable of severing the casing that would normally be run. Consequently, casing drilling with the casing being rotated by casing gripper and top drive to cause the drilling may violate safety regulations in some geographic areas.